I have a Moto E5 Play, which does not have built-in NFC capabilities. I'd like to be able to use Google Pay and other apps that require NFC. Are there NFC transmitters/readers that plug into the 3.5mm audio jack? (Kind of like Square, but with NFC capabilities, not mag stripe) I'd like to leave the USB port free for charging, which is why I'm asking about the audio port.
I don't believe that the thing you're searching for is possible at this time. While there are external NFC addons, they are (as far as I know) designed for taking payments rather than mimicking general NFC android capabilities. (Details and store page for one of these) The problem you run into is that some "flavours" of android (the modified versions used by individual device manufacturers were not designed for NFC, so adding the functionality is nigh on impossible. If you have settings for NFC in your device menus, there might be chance and further investigation could be worth it.
If you do decide to keep investigating, while I couldn't easily find one currently for sale, there are many NFC modules designed for a micro SD slot, however be careful with these as it seemed unclear whether they were intended to add NFC functionality or if they were simply standard micro SD cards which allow a person to access the files on it through NFC.